The present invention relates to industrial and other processes in which large volumes of gases must be pumped at pressures as low as 1-10 milliTorr. Industrial processes in this category include, for example, various types of plasma processing, such as plasma enhanced chemical vapor deposition, plasma mediated etching of surfaces, and other types of surface modification processes.
In many such processes employing plasmas, it is generally considered by those skilled in the art to be advantageous to generate the processing plasmas in suitable mixtures of gases maintained at pressures as low as 1-10 milliTorr. The purity and composition of the gas can best be controlled if the flow rate of fresh gas into the processing chamber is high relative to the processing rate. However, existing vacuum pumping technology can provide only limited throughput of gas in this pressure range.
The pumping speed of widely used turbomolecular vacuum pumps, for example, generally decreases rapidly with increasing pressure at pressures above roughly 1 milliTorr. Robust, cost-effective systems for achieving high-speed pumping in the pressure range from 1-10 milliTorr have not been developed to date.
Furthermore, many gases involved in industrial processing of, for example, VLSI systems are toxic or hazardous and must be isolated and controlled with great care. It would be advantageous if the toxic or hazardous gases to be exhausted from the processing chambers could be converted into less toxic or hazardous forms through dissociation of the molecules of such gases. This process is often referred to as pyrolyzation of toxic or hazardous gases and has been investigated for many years, particularly in connection with toxic gases used by the military.
Conventional vacuum pumping technology utilizes one of two fundamental mechanisms: (1) increasing the momentum of gas molecules in a preferred direction and exhausting the gas molecules through a valve or baffle structure which inhibits the reverse flow of gas; or (2) condensing the gas to be pumped on special surfaces. The first mechanism is usually implemented through some type of piston, blower, or rapidly moving vanes which impart directed momentum to the gas from rapidly moving mechanical structures or streams of pumping molecules, such as mercury or readily condensable pumping oils. The second mechanism is commonly used in systems with low to moderate throughput requirements. In the range of pressures where industrial plasma processes are carried out (1-100 milliTorr), turbomolecular pumps are used almost universally as the first stage of a compound pumping system designed to pump large fluxes (“throughputs”) of process gases.
Turbomolecular pumps impart directed momentum to gas molecules through collisions with rapidly spinning discs. This mechanism is most effective at sufficiently low gas pressures that the mean-free-path of the molecules is larger than the dimensions of the pumping structures. The resulting limit on maximum gas throughput that can be achieved with turbomolecular pumps is a drawback in the plasma processing industry where substantial throughputs of reactant gases are needed to prevent the buildup of reaction products to concentrations that would be deleterious to the process. Further, high-speed turbomolecular pumps are necessarily complex and expensive devices, in which large angular momentum is stored. Moreover, many plasma processes yield solid and/or corrosive byproducts that can be potentially damaging to these pumps.
The ability to pump at 3 to 5 times the presently available pumping speed has been shown (using smaller substrates) to enhance the reliability of the process and performance of etch and deposition processes. The present limit of pumping speed in a properly designed system is determined both by the speed of the pump and the conductance of the chamber to the pump inlet. At present the capability of turbomolecular pumps is limited to 5500 liters per second; although the use of the largest available turbomolecular pumps is further limited by the cost of the large pumps and the expected lack of reliability of a pump this large. The cost of a turbomolecular pump of this size exceeds $80,000, which, at $15.5/(liter/sec), is considerably higher in cost per unit of pumping speed than the $30,000, or $10.6/(liter/sec), for a 3300-liter per second pump. Thus, the cost per unit pumping speed for the larger pumps is 50% greater than for the smaller pumps.
Even so, the 5500 liters/second (1/sec) pump is smaller than would be optimum for processing 200 mm wafers. Extension to 300 mm wafers significantly exacerbates the problem of providing adequate pumping speed. The required pumping speed scales most closely with the area of the substrate. The scaling of process equipment to from 200 mm to 300 mm wafers requires at least a 2.25× increase in pumping speed. The available increase from 3300 liter/second to 5500 liter/second is only an increase of 1.67×. This leaves the 300 mm systems with pumping options that are grossly inadequate.
The problem of gas handling in 300 mm systems is complicated even further by the fact that it is the pumping speed at the wafer (substrate) that is most important. Providing pumping speed at a location remote from the wafer means that the gas atoms must be conducted to the pump inlet through some transitional structure. In the transition region there are invariably reductions in conductance, which is typically measured in liters/sec. Computer programs can predict the overall pumping speed when the gas atoms are in the laminar or molecular flow regimes. In most processes of commercial interest, however, the flow is characterized as transition flow and the computer models are less reliable in predicting performance. The conductance between the wafer and the pump in most designs provides a loss of at least 50%-75% in effective pumping speed.
In addition, the effective handling of gas flow must account for the gas species that must be removed from the system but spend much time attached to the processing chamber walls. A similar problem occurs with any barely volatile species that may result from the process itself, for example, multiple carbon species that are polymerized either by the electrons or photons of the plasma. It is easy for the plasma electron or photon flux to affix these fragments of molecules to the walls. In the same way these materials are subsequently released from the walls, perhaps as a different species. The same processes of attachment, synthesis, decomposition and evolution occur at the substrate but with the difference that the substrate is of different material and has the additional energy flux of the ion bombardment that is used, for example, to promote the etch process. The substrate is a source of organic material and silicon from the etch process. If this flow of new material ceases when the wafer bias is removed the reactor tends to stabilize in that the active groups that are volatile become combined into the degenerate brown film that is often seen in plasma processing systems. This material is highly cross-linked and very stable with respect to thermal desorption or plasma bombardment.
The quality of the process results depends in considerable measure upon the presence or absence of weakly volatile materials such as, for example, in the use of side-wall passivation to provide straight walls in high aspect ratio channels. This passivation results from the deposition of material that originated as photoresist and injected gas and is modified by the electron collisions as a free molecule in the plasma volume and may undergo multiple changes while adsorbed on surfaces. This material is then redeposited on the wafer surface specifically in the freshly etched surface of the feature being etched. The deposited material having experienced multiple interactions with the plasma is particularly resistant to decomposition by the ions and chemically reactive molecules that provide the flux that etches the wafer. Probably much of the process dynamics that allow selectivity between organic surfaces and surfaces that are more inorganic (the plasma tends to spread molecular species to all surfaces in the plasma) are affected by changes in the thin volume of the wall that is in contact with the plasma. For these reasons, if the residence time for these species in the process chamber could be reduced significantly, perhaps shorter that some accommodation time (the time for them to find this stable chemical site), then etch chemistry in the chamber could be controlled and optimized.
For some time there has been a growing appreciation of the possible benefits of using plasmas as the active element in vacuum pumping technologies; for example, plasmas can pump a wide range of gasses, including hydrogen and helium, with equally high efficiencies. Plasma vacuum pumps can be highly tolerant of solid or corrosive process by-products. As described herein, these benefits result from the generic underlying properties of plasma vacuum pumps, in which three-dimensional flow of the neutral gas to be pumped is transformed into one-dimensional flow of a magnetized plasma which can be magnetically compressed and guided through suitable baffle structures. Neutral gas is composed of neutral, i.e. non-ionized, atoms and molecules. Momentum can be imparted to the plasma through various electromagnetic interactions and, in turn, to the neutral gas through collisions between energetic charged particles and neutral gas molecules.
These potential benefits have not yet been fully realized in practice for a number of technical reasons relating to efficient generation of plasma, the creation of a magnetic field suitable for both the plasma generation and the necessary channeling of the plasma flow, and simple and effective mechanisms for driving the plasma flow at pressures in the range of importance to plasma processing applications. This last technical difficulty is exacerbated by the plasma's ability to shield its interior from low-frequency external electric fields, together with the complex atomic and molecular processes that become important in the pressure range of interest.
Another type of plasma vacuum pump is disclosed in International Application No. PCT/US99/12827, filed on Jun. 29, 1999, entitled PLASMA VACUUM PUMPING CELL, the entire disclosure of which is incorporated herein by reference, and pending U.S. Provisional Application No. 60/114,453, filed on Dec. 30, 1998, entitled PLASMA VACUUM PUMP, the entire disclosure of which is incorporated herein by reference. This pump utilizes the plasma excited in a processing system by a high-density plasma source, such as a plasma utilizing electron cyclotron resonance (ECR), an inductively coupled plasma (ICP), or an electrostatically shielded radio frequency (ESRF) plasma, to pump gas out of the system. This type of pumping cell must be designed and built according to the source plasma properties of the system in which it is to be installed. It can not be used as a stand-alone vacuum pump.